Microstructural Analysis of Biodegradable Magnesium Alloy WE43 after Laser Surface Melting

Biodegradable magnesium alloy WE43 (Mg-4Y-3RE) has received great attention in orthopaedic applications as it can dissolve completely after bone tissue repair, eliminating the need for a second surgery to remove the WE43 implant. However, the rapid degradation of WE43 implants during bone healing remains a concern. Rapid degradation can deteriorate the mechanical strength and generate a significant amount of hydrogen gas via corrosion in physiological environments, negatively affecting bone healing and the surrounding tissues. To overcome the rapid degradation of medical implants, one commonly used method is surface modification via laser surface melting (LSM) to alter the surface microstructures and improve the corrosion resistance. This paper investigates the possibility of applying LSM technique to refine the surface microstructures of WE43 alloy and compares the microstructures induced by LSM with the extruded alloy without laser treatment. Results show significant grain refinement after LSM with average grain size decreased to 3μm as compared to 5μm before LSM, approximately 40% reduction in grain size. Different types of grain morphology are also identified at different locations in the melt pool due to different temperature gradients and cooling rates. It is observed that the depth of the melt pool increases with increasing laser power and decreasing laser scanning speed due to the higher heat input. It is also observed that grain size decreases with decreasing laser power and increasing laser scanning speed due to increased cooling rate. Results from this study show that LSM, a form of rapid solidification processing, can form a predominantly basal crystallographic texture, homogenise and refine the surface microstructures of WE43, which are beneficial for corrosion resistance.